Distributed process control systems, like those used in chemical, petroleum or other processes, typically include one or more process controllers communicatively coupled to one or more field devices via analog, digital or combined analog/digital buses. The field devices, which may be, for example, valves, valve positioners, e.g., digital valve positioners, and transmitters, e.g., temperature, pressure, level and flow rate sensors, are located within the process environment and perform process functions such as opening or closing valves and measuring process parameters. Smart field devices, such as the field devices conforming to the well known FOUNDATION® Fieldbus protocol may also perform control calculations, alarming functions, and other control functions commonly implemented within the controller. The process controllers receive signals indicative of process measurements made by the field devices and/or other information pertaining to the field devices. The controller then executes a controller application that runs, for example, different control modules which make process control decisions, generate control signals based on the received information and coordinate with the control modules or blocks being performed in the field devices, such as HART and Fieldbus field devices. The control modules in the controller send the control signals over the communication lines to the field devices to thereby control the operation of the process plant.
More specifically, a process control network or system includes one or more process controllers connected to one or more host workstations or computers (which may be any type of personal computer, workstation or other computer) and to a data historian via a communication connection. The communication connection may be, for example, an Ethernet communication network or any other desired type of private or public communication network. Each of the controllers is connected to one or more input/output (I/O) devices each of which, in turn, is connected to one or more field devices, such as a digital valve positioner. As one of skill in the art will appreciate, the process control system could include any other number of controllers and any desired number and types of field devices. Of course, the controllers are communicatively connected to the field devices using any desired hardware and software associated with, for example, standard 4-20 ma devices and/or any smart communication protocol such as the Fieldbus or HART protocols. As is generally known, the controllers, which may be, by way of example only, DeltaV™ controllers sold by Fisher Rosemount Systems, Inc., implement or oversee process control routines or control modules stored therein or otherwise associated therewith and communicate with the devices to control a process in any desired manner.
As noted, an exemplary field instrument of the process control system is the digital valve positioner. As is well known to persons having ordinary skill in the art, the digital valve positioner converts an input current signal into a pneumatic output pressure to an actuator to which the digital valve positioner is operatively coupled. In addition to this normal function, the digital valve positioner also uses a communications protocol, such as the HART communications protocol, to give easy access to information critical to the process operation. In one example, the digital valve positioner provides comprehensive valve diagnostic alerts that are easily accessed via a field communicator and delivers notification of current or potential equipment issues to an asset management system. For example, the alerts assist in identification and notification of several situations including: (1) valve travel deviation due to excess valve friction or galling; and (2) valve travel above or below a specified point. The alerts are stored in a memory on board the digital valve positioner.
One trend in the digital valve positioner market specifically, and the electro-pneumatic instrument market generally, is to design instruments with significantly greater flow capacities than conventional instruments. For example, conventional digital valve positioners typically have a flow coefficient (Cv) of 0.3, while higher flow capacity digital valve positioners have a flow coefficient (Cv) ranging from 1.2 to 6.4. This higher flow capability means that the pneumatic porting in the product, e.g., supply porting, output porting, and exhaust porting, has to be significantly larger than conventional digital valve positioners, for example.
Such larger porting, however, can lead to a greater chance for dirt and moisture and any other external medium in the environment to migrate into the digital valve positioner, which can adversely affect the operation of internal devices of the digital valve positioner. Specifically, the exhaust ports of the digital valve positioner are prime areas where dirt and moisture and other external media can enter the digital valve positioner. In addition, higher flow rates of the higher capacity digital valve positioner generate more noise, such that the noise levels are directly increasing in proportion to the increasing flow rate.